The second prototype Grumman/General Dynamics F-111B, BuNo 151971, c/n A2-02, crashed into the Pacific Ocean killing Hughes pilot Barton Warren and his RIO Anthony Byland. 
The F-111B was developed in the 1960s by General Dynamics in conjunction with Grumman for the U.S. Navy as part of the joint Tactical Fighter Experimental (TFX) with the United States Air Force (USAF) to produce a common fighter for the services that could perform a variety of missions. It incorporated innovations such as variable-geometry wings, afterburning turbofan engines, and a long-range radar and missile weapons system.
F-111B #151971 was a prototype with a heavy airframe and TF30-P-3 engines. This plane was used for Hughes missile testing before it was lost in the crash of September 11, 1968.
Source: Wikipedia
